In Magnetic Resonance Imaging (MRI) and in particular cardiovascular MRI, “T2-preparation” or “T2-prep” is used to magnetically prepare tissue, for example myocardium (heart tissue), to create image contrast between different tissue types due to their differences in T2. This is also known as T2-weighting. T2 is a time constant describing the decay of transverse magnetization and is a function of local tissue water content, among other parameters. The shorter the tissue T2 value in a region of interest (ROI) the darker the ROI appears in a T2-weighted MR image, and conversely, the longer the T2 in a ROI, the brighter the tissue in that ROI. This allows clinicians to discriminate abnormal regions that have a relatively long T2 value (such as edema), from healthy regions possessing a relatively shorter T2 value. In the heart for example, regions with long T2 are known to occur in the setting of acute myocardial infarction. T2-preparation is also used in coronary angiography to accentuate the signal difference between myocardium and blood. Whereas myocardium is rendered dark by T2-preparation, blood remains bright, improving the image contrast between myocardium and blood. FIG. 1 shows a short-axis T2-weighted image of a canine heart indicating elevated image intensity 103 as a result of edema.
T2-preparation sequences use an initial tip-down radio frequency (RF) pulse to convert a substantial part of the longitudinal magnetization of the imaged volume to transverse magnetization, a combination of time delays and RF pulses designed to refocus this transverse magnetization after some signal decrease through T2 relaxation during these pulses and delays, followed by a final tip-up RF pulse to return a substantial part of the refocused magnetization to longitudinal magnetization. The T2 relaxation between the tip-down and tip-up pulses provides the desired alteration of image contrast between components of the imaged volume with different T2 relaxation rates.
One type of known T2-preparation method, MLEV (Levitt and Freemann 1981; Levitt, Freemann et al, 1982; Brittain, Hu et al. 1995), is adversely affected by inhomogeneities of the MRI magnetic excitation field B1 and/or the static magnetic field B0. These inhomogeneities are exacerbated with increasing field strength. Other known types (Nezafat, Stuber et al. 2006; Nezafat, Derbyshire et al. 2008; Nezafat, Ouwerkerk et al. 2009; Nezafat, Ouwerkerk et al. 2010) are susceptible to motion and blood flow resulting in signal variations known as image inhomogeneity, and in image artifacts within the imaged volume. Specifically for moving organs such as the heart, resulting signal variations across the myocardium can be mistaken for intensity changes due to patho-physiology.
Known MLEV composite pulses Levitt, Freemann et al. 1982; Brittain, Hu et al. 1995) can partially compensate for imperfections in the RF magnetic field B1, but fail to yield a homogeneous tissue preparation at field strengths of 3 T (Tesla) or higher (Rehwald, Jenista et al. 2011).
An improved compensation for imperfections in the RF magnetic field B1 can be achieved with so called adiabatic RF pulses. Adiabatic pulses combine amplitude and frequency modulation of the RF designed to create a rotation of the magnetization in a way that is insensitive to variations of the RF field (B1) strength over a substantial range of RF field strengths.
Known T2-preparations that partially or exclusively employ adiabatic RF pulses are a) a matched pair of adiabatic inversion recovery (IR) pulses (Nezafat, Stuber et al. 2006; Nezafat, Ouwerkerk et al. 2010), and b) a single deconstructed BIR4 (deconstructed B1-insensitive rotation with 4 segments, dBIR4) (Nezafat, Derbyshire et al. 2008; Nezafat, Ouwerkerk et al. 2009).
The matched IR pair method consists of an adiabatic IR pulse followed by a time delay to allow magnetization to evolve, followed by a second identical adiabatic IR pulse. A matched pair of identical adiabatic IR pulses is required for refocusing by IR pulses to work (Nezafat, Stuber et al. 2006; Nezafat, Ouwerkerk et al. 2010), but this requirement is problematic as it may make the method susceptible to motion and flow. Phase errors introduced by the first inversion pulse can only be fully compensated by the second inversion in the absence of motion and flow. With motion and flow present, the resulting tissue preparation, e.g. in the heart, is not homogeneous and blood flow creates artifacts. The longer the time delay between the IR pulses the more the module becomes sensitized to the dephasing effects of motion and blood flow, and the more degraded its image quality.
The dBIR4 module is affected by motion and blood flow causing major artifacts and signal inhomogeneity especially for longer T2-preparation times (above 40 ms) (Rehwald, Jenista et al. 2011). Analogous to described matched IR pair problems, this is likely due to the inability of dBIR4 to fully compensate for phase errors in the presence of motion and flow. This inability increases with larger delays between the adiabatic pulses.
A system according to invention principles addresses the above limitations and has excellent robustness in the presence of motion, flow as well as B1- and B0-inhomogeneity.